The present invention relates to transducers of the type used to produce a sonic output. More specifically, the present invention relates to controlling the sonic output from a transducer using a feedback technique.
Sonic transducers, and in particular ultrasonic transducers, are used in a wide variety of applications to provide a sonic output. For example, ultrasonic transducers are used for imaging, medical therapy, motors, sonar systems, welding, cleaning, instrumentation, chemical activation, machining and vaporizing. One example use in the medical field is in the Copalis(copyright) testing system available from DiaSorin Inc. of Stillwater, Minn. In the Copalis(copyright) testing system, an ultrasonic transducer is used for resuspension of particles in a fluid.
One problem commonly associated with ultrasonic transducers is the inability to accurately control the energy delivered by the ultrasonic transducer. This is largely due to the inability to accurately determine the energy level of the ultrasonic output provided by a drive element in the transducer. This has made it difficult to accurately ascertain whether the ultrasonic transducer is providing the desired level of ultrasonic energy to the work piece.
One technique used to overcome the problem of controlling the output is to accurately calibrate the transducer prior to use. However, the output energy level is dependent upon a number of different factors and can experience drift during operation. For example, a change in the force applied to the transducer can affect the energy output. The delivered energy level is also affected by factors such as drive voltage, ambient temperature, temperature rise due to self heating of the transducer during operation, and a change in the resonant frequency of the transducer. This problem is exacerbated because the ultrasonic transducer must operate in the stable and desired frequency regimes in order to operate efficiently.
One technique for automatically controlling the drive signal frequency applied to an ultrasonic transducer is to compare the phase of the drive voltage signal to the phase of the drive current signal. When the voltage and current signals are in phase, the ultrasonic transducer is operating at a resonant frequency. However, this technique is complex, inefficient, and does not provide a direct indication of the amount of energy in the ultrasonic transducer. Another technique is to use a separate sensor spaced apart from the ultrasonic transducer to monitor the energy output. However, this technique is sensitive to standing waves which may cause inaccurate readings. Further, this technique can be inaccurate due to interfacial changes between materials.
Other techniques of controlling the transducer use a sense element to determine if the transducer is operating at resonance. Such techniques are described in, for example, U.S. Pat. No. 3,889,166, issued Jun. 10, 1975, and entitled AUTOMATIC FREQUENCY CONTROL FOR A SANDWICH TRANSDUCER USING VOLTAGE FEEDBACK; U.S. Pat. No. 4,197,478, issued Apr. 8, 1980, and entitled ELECTRONICALLY TUNABLE RESONANT ACCELEROMETER; U.S. Pat. No. 4,728,843, issued Mar. 1, 1988, and entitled ULTRASONIC VIBRATOR AND DRIVE CONTROL METHOD THEREOF; U.S. Pat. No. 4,441,044, issued Apr. 3, 1984, and entitled TRANSDUCER WITH A PIEZOELECTRIC SENSOR ELEMENT; and U.S. Pat. No. 5,536,963, issued Jul. 16, 1996, and entitled MICRODEVICE WITH FERROELECTRIC FOR SENSING OR APPLYING A FORCE. Although above mentioned techniques describe the use of a separate sense element to detect if the transducer is operating at a mechanical resonant frequency, these techniques have not monitored and controlled the energy level of the transducer.
A sonic transducer includes a transducer body and a sonic drive element coupled to the transducer body to produce a sonic output in response to an applied electrical input. An electromechanical transducer such as a sonic transducer includes a transducer body and an electromechanical drive element coupled to the transducer body to produce an electromechanical output, such as a sonic output in response to an applied electrical input. A sense element is coupled to the drive element and is configured to provide an electrical feedback output related to the electromechanical output. The electrical feedback output is adapted to be used to control the applied electrical input to the drive element.